Hi and welcome!
Knit until you like the length around your neck. The sample in the photos was 20 inches long before it was seamed. When you join the cast on and the last row, yes, that'll make a circle for your head.
I'm not sure what you mean by divide the cast on stitches in two? Then you won't have a circle. What would you do with the cast off sts?
I like this pattern. It's such soft cables. I've used Cascade Venezia before and it is a dream to work with (I don't know if that's the yarn you're working with.)